pass MYSQL_PWD to proc_open(), instead of altering the current environment
Refactor MySQL utilities:

* extract mysql_host_to_cli_args() helper
* make run_mysql_command() accept an associative array of args
* remove redundant run_mysql_query() utility
minor cleanup in mysql_host_to_cli_args()
Merge pull request #685 from wp-cli/db-call-cleanup

Refactor code that makes calls to `mysql` and friends